Help! first tube pre-amp with 12ax7

zafira1981 · 8th December 2006, 10:10 PM

Hi!!!

Im kind of new with tubes so I decided to make my first design of an audio amplifier.

I have to design a pre for a kt88 push-pull output. I want to use 12ax7+12au7

So, the first stage is a 12ax7 voltage amplifier.

The B+ power is 540V. I took the datasheet and decided to bias the 12ax7 with

Vp=250V
Vg=-2V
Ip=1.2mA

The RP with that bias is 62500ohms

Rk = 2v / 1.2ma = 1K66 ohms
RA = (540-2-250)/1.2mA = 240k -> 220K

The bias is working fine.

AV = (mu Ra)/(Rp+Ra)=77

I have lot of .47uFx630V caps so I used that for signal decoupling, and a 22uF for Rk

here is the schematics:

It works OK at 1khz, the gain is almost 77 but I made some measurements (with a true-RMS voltage/frequency meter) measuring Vin and Vout, and here is the results

Code:

f(hz)	vo/vi
10	66,25
19	70,72
28	72,97
46	74,27
70	74,37
250	75,36
700	72,57
1000	74,04
4500	47,66
10000	27,46
15000	21,76
18000	18,50
20000	16,69
30000	12,94

My question is, why do I have that high-frequency cut at higher frequencies than 1khz???

thanks!!!

Jorge.

SY · 8th December 2006, 10:27 PM

You have a high source impedance and are loading the circuit down with the input capacitance of your meter (probably 100pF or a bit more) along with all the strays of the wiring. Try it again with a 10x probe.

zafira1981 · 8th December 2006, 10:37 PM

thanks for your reply.

I'm using a protek 506 multimeter
http://ee.fatih.edu.tr/ComLab/images/protek506.jpg

Well I will have to wait for the scope that I'm planning to buy.

thanks again!!!

8th December 2006, 10:10 PM	#1
zafira1981 diyAudio Member Join Date: Jan 2005 Location: Argentina	Help! first tube pre-amp with 12ax7 Hi!!! Im kind of new with tubes so I decided to make my first design of an audio amplifier. I have to design a pre for a kt88 push-pull output. I want to use 12ax7+12au7 So, the first stage is a 12ax7 voltage amplifier. The B+ power is 540V. I took the datasheet and decided to bias the 12ax7 with Vp=250V Vg=-2V Ip=1.2mA The RP with that bias is 62500ohms Rk = 2v / 1.2ma = 1K66 ohms RA = (540-2-250)/1.2mA = 240k -> 220K The bias is working fine. AV = (mu Ra)/(Rp+Ra)=77 I have lot of .47uFx630V caps so I used that for signal decoupling, and a 22uF for Rk here is the schematics: It works OK at 1khz, the gain is almost 77 but I made some measurements (with a true-RMS voltage/frequency meter) measuring Vin and Vout, and here is the results Code: f(hz) vo/vi 10 66,25 19 70,72 28 72,97 46 74,27 70 74,37 250 75,36 700 72,57 1000 74,04 4500 47,66 10000 27,46 15000 21,76 18000 18,50 20000 16,69 30000 12,94 My question is, why do I have that high-frequency cut at higher frequencies than 1khz??? thanks!!! Jorge.

8th December 2006, 10:27 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX	You have a high source impedance and are loading the circuit down with the input capacitance of your meter (probably 100pF or a bit more) along with all the strays of the wiring. Try it again with a 10x probe. __________________ "It is not seemly, after wiping your nose, to spread out your handkerchief and peer into it as if pearls and rubies might have fallen out of your head." - Erasmus

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8th December 2006, 10:37 PM	#3
zafira1981 diyAudio Member Join Date: Jan 2005 Location: Argentina	thanks for your reply. I'm using a protek 506 multimeter http://ee.fatih.edu.tr/ComLab/images/protek506.jpg Well I will have to wait for the scope that I'm planning to buy. thanks again!!!

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